Add an I64Regs register class for 64-bit registers.

We are going to use the same registers for 32-bit and 64-bit values, but
in two different register classes. The I64Regs register class has a
larger spill size and alignment.

The addition of an i64 register class confuses TableGen's type
inference, so it is necessary to clarify the type of some immediates and
the G0 register.

In 64-bit mode, pointers are i64 and should use the I64Regs register
class. Implement getPointerRegClass() to dynamically provide the pointer
register class depending on the subtarget. Use ptr_rc and iPTR for
memory operands.

Finally, add the i64 type to the IntRegs register class. This register
class is not used to hold i64 values, I64Regs is for that. The type is
required to appease TableGen's type checking in output patterns like this:

  def : Pat<(add i64:$a, i64:$b), (ADDrr $a, $b)>;

SPARC v9 uses the same ADDrr instruction for i32 and i64 additions, and
TableGen doesn't know to check the type of register sub-classes.
Add support for 64-bit calling convention.

This is far from complete, but it is enough to make it possible to write
test cases using i64 arguments.

Missing features:
- Floating point arguments.
- Receiving arguments on the stack.
- Calls.
Add predicates for distinguishing 32-bit and 64-bit modes.

The 'sparc' architecture produces 32-bit code while 'sparcv9' produces
64-bit code.

It is also possible to run 32-bit code using SPARC v9 instructions with:

  llc -march=sparc -mattr=+v9
Add 64-bit shift instructions.

SPARC v9 defines new 64-bit shift instructions. The 32-bit shift right
instructions are still usable as zero and sign extensions.

This adds new F3_Sr and F3_Si instruction formats that probably should
be used for the 32-bit shifts as well. They don't really encode an
simm13 field.
Materialize 64-bit immediates.

The last resort pattern produces 6 instructions, and there are still
opportunities for materializing some immediates in fewer instructions.
Basic 64-bit ALU operations.

SPARC v9 extends all ALU instructions to 64 bits, so we simply need to
add patterns to use them for both i32 and i64 values.
Add 64-bit load and store instructions.

There is only a few new instructions, the rest is handled with patterns.